Jominy hardenability and D_I

Hardenability is about depth

When steel is quenched, the surface cools fastest and the core slowest. Martensite only forms where the cooling rate beats the diffusional (pearlite/bainite) transformation. Carbon sets how hard that martensite is; the alloying elements — manganese, chromium, molybdenum, nickel, boron — set how slow a cooling rate can still avoid the diffusional nose, and therefore how deep hardening reaches.

The Jominy end-quench

ASTM A255 standardises the measurement: austenitise a 25 mm bar, quench water onto one end only, then traverse hardness along the length. Cooling rate decreases monotonically with distance from the quenched end, so each Jominy distance corresponds to a known cooling rate. A flat, slowly-falling curve means high hardenability; a curve that drops away quickly means low.

The ideal critical diameter

To turn composition into a single number, Grossmann's method computes the ideal critical diameter — the bar that would just be 50% martensite at centre under a perfect quench — as a base value (from carbon and austenite grain size) multiplied by an alloy factor for each element:

Using it in design

Match hardenability to section size: a shaft that must be through-hardened needs a steel whose D_I exceeds its diameter for the available quench severity, while a part that only needs a hard case can use a leaner, cheaper, less-distortion-prone grade. The Jominy curve also feeds directly into predicting microstructure and properties at any depth via the local cooling rate.